1. Field of the Invention
The present invention relates to an optical fiber and an optical device employing the optical fiber.
2. Description of the Related Art
In recent years, a laser light source that outputs a high-power laser light having a wavelength near 1000 nanometers has been matured, which includes yttrium aluminum garget (YAG) laser having an oscillation wavelength of 1064 nanometers and an optical fiber laser using a ytterbium-doped optical fiber (YDF) having an oscillation wavelength of 1100 nanometers. With the development of such a laser light source, a nonlinear optical device is getting an attention, such as a supercontinuum light generation device using the laser light source as a pumping light source. An optical fiber used in such a nonlinear optical device is required to have the zero dispersion wavelength near the wavelength of the pumping light with high optical nonlinearity.
In the case of a generally-used single-mode optical fiber, which is made of silica-based glass, the wavelength dispersion characteristic is represented by a sum of the structure dispersion and the material dispersion. The structure dispersion is a wavelength dispersion determined by the refractive index profile of the optical fiber, and the material dispersion is a wavelength dispersion determined by the optical characteristics of the silica-based glass that is the main constituent material of the optical fiber. Because the characteristic of the structure dispersion can be largely changed by changing the refractive index profile of the optical fiber, it is possible to set the zero dispersion wavelength at which the wavelength dispersion becomes zero to a desired wavelength by adjusting the structure dispersion of the optical fiber.
Nevertheless, the wavelength where the zero dispersion wavelength can be easily set by adjusting the structure dispersion is equal to or longer than 1200 nanometers, and it is difficult to set the zero dispersion wavelength to a wavelength shorter than 1200 nanometers. The reason is as follows. The material dispersion of the silica-based glass has a positive value at a wavelength side longer than a wavelength of about 1300 nanometers, showing an anomalous dispersion. However, it becomes negative at a wavelength of about 1300 nanometers, showing a larger normal dispersion as the wavelength becomes shorter at a region of a short wavelength side. On the other hand, the structure dispersion is basically a normal dispersion at a wavelength side longer than about 1000 nanometers, and even if the structure dispersion is shifted to the anomalous dispersion side by changing the refractive index profile, the absolute value of the dispersion is small. For this reason, the wavelength dispersion of the optical fiber, which is the sum of the material dispersion and the structure dispersion, becomes negative. As a result, it is difficult to set the zero dispersion wavelength to a wavelength shorter than 1200 nanometers.
Meanwhile, an optical fiber commonly known as a photonic crystal fiber has been reported, which has a large number of holes around the core region made of silica glass. Regarding the photonic crystal fiber, there are reports saying that the structure dispersion with a larger absolute value can be obtained (see T. A. Birks, et al., “Dispersion compensation using single-material fibers”, Photon. Tech. Lett., 11, p 674 (1999) and J. C. Knight, et al., “Anomalous dispersion in photonic crystal fiber”, Photon. Tech. Lett., 12, p 807 (2000)). In addition, another report says that a single-mode operation and the zero dispersion wavelength characteristic can be obtained at any wavelength by using the structure obtained by forming such holes in the photonic crystal fiber (see P. J. Bennett, et al., “Toward practical holey fiber technology: fabrication, splicing, modeling, and characterization”, Opt. Lett., 24 p 1203 (1999)). In the photonic crystal fiber, the refractive index profile of the optical fiber can be largely changed by arranging as large number of holes as 60 to 300 in the cladding region, which makes it possible to obtain the structure dispersion with a large absolute value. With this mechanism, for example, a large anomalous dispersion can be obtained at a short wavelength region of the near infrared, and therefore the wavelength dispersion can be made zero by combining with the material dispersion having a large normal dispersion. In the photonic crystal fiber, the wavelength dispersion characteristic largely depends on the size of the holes and the accuracy of arranging the holes. However, because it is difficult to fabricate the optical fiber in which such a larger number of holes are accurately arranged, the manufacturing yield becomes decreased resulting in an increase of cost. Furthermore, in the photonic crystal fiber, a dopant for increasing the refractive index, such as germanium, is not doped in the core region, and the effective refractive index of the core region is low, which easily causes a large optical leakage loss. Because a large number of hole layers are necessary to suppress the leakage loss, the total number of holes cannot be reduced, which causes a further decrease of the manufacturing yield and a further increase of cost.
On the other hand, an optical fiber in which a number of holes are arranged around a core region doped with the germanium has been reported, which is called the hole-assisted fiber (see T. Hasegawa, et al., “Novel hole-assisted lightguide fiber exhibiting large anomalous dispersion and low loss below 1 dB/km”, OFC 2001, D5-1 (2001)). The features of the hole-assisted fiber are: it is possible to reduce the macro-bending loss because a strong optical confinement in the core region can be obtained by providing the holes; and the structure dispersion can be largely changed by providing the holes near the core region. In addition, unlike the photonic crystal fiber, because the hole-assisted fiber includes a core region having a refractive index higher than that of a cladding region, the effective refractive index of the core region is higher than the refractive index of the cladding region, which makes it possible to easily suppress the optical leakage loss even without providing a large number of hole layers.
However, regarding a single-mode optical fiber that has the zero dispersion wavelength at a wavelength near 1000 nanometers and is particularly suitable for a nonlinear optical device in this wavelength band, the photonic crystal fiber is not a good candidate because it is difficult to fabricate the fiber, which increases the cost. Furthermore, in the case of the hole-assisted fiber, the shift of the zero dispersion wavelength to a wavelength near 1000 nanometers by controlling the structure dispersion increases the optical confinement, which leads the light to easily propagate in a higher-order mode, and therefore a single-mode operation at the zero dispersion wavelength becomes hard to obtain.